1. Field of the Invention
The present invention relates to an optical switch, and in particular to an optical switch comprising a plurality of input ports and output ports, and performing a path establishment between the input ports and the output ports.
With a recent traffic growth, an increase in network capacity has been demanded. Therefore, construction of an optical network based on Wavelength Division Multiplexing (WDM) technology has been required in a backbone network.
The WDM technology increases a point-to-point transmission capacity by transmitting a plurality of optical signals having different wavelengths on a single optical transmission line. Also, for applications of the WDM technology, there are cited an optical switch apparatus such as an add-drop multiplexer adding/dropping information of a specified wavelength, and an optical cross-connect switching over a transmission line per optical wavelength.
In such an optical switch apparatus, an optical switch to perform a path switchover of an optical signal per wavelength plays an important part.
2. Description of the Related Art
FIG. 13 shows an arrangement (1) of an optical switch apparatus (optical cross-connect) 100 including a general optical switch (routing portion) 20. The optical switch apparatus 100 accommodates a plurality of input optical transmission lines 1_1-1_m (hereinafter, occasionally represented by a reference numeral 1) and output optical transmission lines 2_1-2_m (hereinafter, occasionally represented by a reference numeral 2), and routes wavelength-multiplexed optical signals coming from the input optical transmission lines 1 to the desired output optical transmission lines 2 per wavelength. Also, an operation system 41 shown in FIG. 13 monitors/controls the optical switch apparatus 100 to perform a path establishment and a path switchover.
The optical switch apparatus 100 is composed of branching portions 10_1-10_m (hereinafter, occasionally represented by a reference numeral 10) branching the wavelength-multiplexed optical signals (wavelengths: xcex1, xcex2, . . . , xcexn) coming from the input optical transmission lines 1_1-1_m, the optical switch (routing portion) 20 routing the optical signals inputted from the input ports to the desired output ports, wavelength converters 31_11-31_1n, . . . , 31_m1-31_mn (hereinafter, occasionally represented by a reference numeral 31) converting the wavelengths of the inputted optical signals into desired wavelengths, and couplers 30_1-30_m (hereinafter, occasionally represented by a reference numeral 30) coupling the optical signals whose wavelengths are converted.
For examples of the wavelength converter 31, there are cited a method of converting a wavelength in the state of light by making use of a Semiconductor Optical Amplifier (SOA), a method of converting a wavelength by making use of a light-electricity converter and an electricity-light converter, and the like. Also, the branching portion 10 and the coupler 30 can be composed of elements using an Arrayed Waveguide Grating (AWG) and a dielectric multilayer film.
FIG. 14 shows an arrangement (2) of the optical switch apparatus (optical cross-connect) 100 including the optical switch. In this arrangement (2), light reproducers 11_11-11_1n, . . . , 11_m1-11_mn are inserted at the preceding stage of the optical switch 20 in the optical switch apparatus 100 shown in FIG. 13, and light reproducers 32_11-32_1n, . . . , 32_m1-32_mn, which also serve as wavelength converters, are arranged instead of the wavelength converters 31 at the subsequent stage.
The light reproducers 11_11-11_1n, . . . , 11_m1-11_mn are thus provided because the optical cross-connect 100 is generally deployed in a long-distance network in many cases and an optical signal waveform inputted to the optical cross-connect 100 deteriorates to the extent that the signal with the original quality can not be reproduced only with an amplification of an optical amplifier.
Also, for example of the optical switch 20 shown in FIGS. 13 and 14, there are cited a waveguide-type switch utilizing a thermal optical effect, a mechanical-type switch utilizing a motor, and the like.
FIGS. 15A and 15B show an arrangement of the optical switch 20 using switch elements 21_1-21_16 (hereinafter, occasionally represented by a reference numeral 21) of a Mach-Zehnder interference-type which is the waveguide-type switch.
The optical switch element 21 is a two-input-two-output-type switch having input terminals 5_1 and 5_2, and output terminals 6_1 and 6_2. When the element 21 is on, the input terminal 5_1 and the output terminal 6_1 are connected, and the input terminal 5_2 and the output terminal 6_2 are connected respectively. When the element 21 is off, the input terminal 5_1 and the output terminal 6_2 are connected, and the input terminal 5_2 and the output terminal 6_1 are connected respectively.
Although the optical switch 20 in FIG. 15A is different from that in FIG. 15B for the connection method of the optical switch elements 21, both switches comprise a four-input-four-output optical switch 20 which connect input ports 3_1-3_4 to output ports 4_1-4_4 in a one-to-one relationship.
In case a path is established between the input port 3_1 and the output port 4_2 for example, the optical switch 20 in FIG. 15A sets the optical switch elements 21_4, 21_3, 21_6, 21_10, and 21_14 off, and sets only the optical switch element 21_2 on. In the optical switch 20, the numbers of the optical switch elements through which the paths pass are not equal.
On the other hand, the optical switch 20 in FIG. 15B sets the optical switch elements 21_1, 21_6, and 21_14 off, and sets only the optical switch element 21_11 on. This optical switch 20 is called PI-LOSS composition, where the number of the optical switch elements 21 through which each path passes is 4, so that optical losses on the paths are basically equal.
These optical switches 20 have problems as follows:
(1) Crosstalk occurs in the optical switches 20, so that a crosstalk signal has a bad influence on an optical signal;
(2) Since the number of the optical switch elements required by the optical switches 20 increases in proportion to the square of the number of the input/output ports and the insertion loss increases, it is difficult to enlarge the scale.
The problem (1) will be first described.
FIG.16 illustrates crosstalks which occur in the above-mentioned four-input-four-output optical switch 20. When the path is established between the input port 3_1 and the output port 4_2 and an optical signal S is transmitted through this path as shown, the optical signal S simultaneously leaks to the output ports 4_1, 4_3, and 4_4, so that crosstalks C1, C2, and C3 occur.
Crosstalks caused by the optical signals of other paths, which are similar to these crosstalks C1-C3, also occur at the output ports 4_1-4_4. All of the crosstalks are overlapped per output port, which forms the crosstalk of each output port.
For a solution of the problem (1), a crosstalk shutdown apparatus mentioned in the Japanese Patent Application Laid-open No.11-41636 is composed so that a crosstalk which propagates through the input port and the output port, and a crosstalk within the optical signal are detected at the input port and/or output port, are intercepted, thereby passing only the optical signal.
The problem (2) will be described.
For the solution of the problem (2), N inputxc3x97N output optical switch 20 using 2N (N=16 in FIG. 17) movable mirrors as shown in FIG. 17 has been proposed. In this optical switch 20, the number of the mirrors increases in proportion to the number of the input/output ports. Accordingly, since neither the number of the mirrors (the number of the switch elements) increases nor the insertion loss increases, compared with the optical switch 20 shown in FIG. 15, the N inputxc3x97N output optical switch 20 is considered suitable for enlarging the scale.
The movable mirror-type optical switch 20 is composed of input optical fibers 22_1-22_16 (hereinafter, occasionally represented by a reference numeral 22), input movable mirrors 24_1-24_16 (hereinafter, occasionally represented by a reference numeral 24) corresponding to the input optical fibers 22, output optical fibers 27_1-27_16 (hereinafter, occasionally represented by a reference numeral 27), and output movable mirrors 25_1-2516 (hereinafter, occasionally represented by a reference numeral 25) corresponding to the output optical fibers 27.
The optical signal inputted from the input optical fiber 22_3, for example, is deflected (reflected) at the movable mirrors 24_3 and 25_14 to be transmitted to the output optical fiber 27_14.
The optical switch 20 is provided with a controller (not shown) controlling the angles of the movable mirrors 24 and 25 in order to establish arbitrary paths between the input optical fibers 22 and the output optical fibers 27.
In such an optical switch 20, the crosstalk described referring to FIG. 16 assumes an accumulated value of leaked lights from other paths, which can be usually neglected if a feedback control is performed to the direction of a desired movable mirror.
However, when the optical signal from the input optical fiber 22_3 is switched over from the output optical fiber 27_14 to the output optical fiber 27_1 for example, upon a path establishment or an occurrence of a transmission line fault, the optical signal sometimes passes through the output movable mirror 25 used for another path during the switchover, so that the crosstalk (crosstalk during the switchover) occurs at this time, resulting in a bad influence on a signal quality.
FIG. 18 shows a state of a crosstalk during switchover which occurs at this time. In the optical switch 20, a path P1 is established between the input optical fiber 22_1 and the output optical fiber 27_k, while a path P2 is established between the input optical fiber 22_16 and the output optical fiber 27_16.
When the path P2 is switched over to a path P3 between the input optical fiber 22_16 and the output optical fiber 27_1 for example, a crosstalk C occurs in the optical fiber 27_k during the switchover, which has a bad influence on the quality of the optical signal of the path P1. Especially, when the wavelengths of the paths P1 and P2 are the same xcexn, bad influence on the quality of the optical signal caused by the crosstalk C which occurs on the output optical fiber 27_k is significant.
Furthermore, when the optical switch 20 is an optical cross-connect, there is a possibility that paths corresponding to the number of transmission line wavelengths simultaneously perform switchover operations (or establishment operations), thereby increasing the possibility of the crosstalk C occurrence.
FIGS. 19A-19C show that the crosstalk from the input movable mirror 24 adjoining the input movable mirror 24 is the largest.
In FIG. 19A, the path P1 through the movable mirrors 24_1 and 25_1 is established between the input optical fiber 22_1 and the output optical fiber 27_1, while the path P2 through the movable mirrors 24_2 and 25_2 is established between the input optical fiber 22_2 and the output optical fiber 27_2.
In FIG. 19B, when the path P1 is switched over to the path P3 (see FIG. 19C) between the input optical fiber 22_1 and the output optical fiber 27_3, the optical signal from the input optical fiber 22_1 is deflected at the output movable mirror 25_2, during the switchover, to be outputted to the output optical fiber 27_2 as a crosstalk light.
As for the crosstalk light, the crosstalk of the optical signal deflected from the input movable mirror 24_1 (or 24_3) adjoining the input movable mirror 24_2 becomes the largest. This is because the angle difference between the signal light and the crosstalk light becomes minimum.
In case such a crosstalk upon a switchover (or establishment) occurs at the optical signal on the output optical fiber (output port) side, the above-mentioned crosstalk shutdown apparatus can not shut down or intercept the crosstalk.
It is accordingly an object of the present invention to provide an optical switch comprising a plurality of input ports and output ports, performing a path establishment between the input ports and the output ports, and reducing a crosstalk which occurs upon the path establishment or a path switchover.
In order to achieve the above-mentioned object, an optical switch of the present invention comprises: a plurality of input ports, a plurality of output ports, and interception means for intercepting an optical signal at a preceding stage of the input ports during a path switchover.
Namely, in case of a path establishment, a path switchover, a connection switchover upon a fault, and the like for example, an optical switch performs a path switchover so that optical signals inputted from input ports may be outputted to any of output ports.
Interception means intercept the optical signal inputted to the optical switch during the path switchover, i.e. from the start of the path switchover to the end thereof Thus, during the path switchover, a crosstalk caused by the optical signal does not occur within the optical switch.
As the interception means, an optical switch element provided between the optical switch and a light source of the optical signal may be used.
Also, as the interception means, an optical amplifier provided between the optical switch and a light source of the optical signal may be used and by decreasing a gain of the optical amplifier, for example, during the path switchover, the optical signal may be intercepted.
Also, as the interception means, a controller which turns on/off a light source of the optical signal may be used, so that by turning off the light source no optical signal may be intercepted to provide an intercepted state.
Also, as the interception means, an optical modulator modulating the optical signal may be used, and by turning off a modulation driver for example, the outputted optical signal may be intercepted.
Also, as the interception means, a collimator controller shifting a focus of a collimator arranged on an input port side may be used, so that the optical signal inputted to the optical switch may be out of focus, scattered, and almost intercepted.
It is to be noted that the optical switch may comprise a movable mirror-type optical switch performing the path switchover with movable mirrors.
Furthermore, the present invention comprises: a plurality of input ports, a plurality of output ports, movable mirrors establishing paths between the input ports and the output ports, and a controller controlling the movable mirrors, during a path switchover, so as to prevent optical signals deflected by the movable mirrors from being outputted to all of the output ports except an output port for a new path establishment.
Also, in the present invention, the controller may control a first movable mirror, during the path switchover, so as to prevent an optical signal reflected by the first movable mirror from being entered into other movable mirrors except a second movable mirror necessary for a new path establishment.
Namely, the optical switch sequentially makes the optical signal inputted from the input port reflected by the first and the second movable mirror to be outputted to a predetermined output port.
Therefore, the controller controls the first movable mirror, during the path switchover, so as to prevent the optical signal reflected by the first movable mirror from being entered into the other movable mirrors except the second movable mirror necessary for a new path establishment.
Thus, the optical signal is not outputted from the output ports except the predetermined output port for the new path establishment during the switchover, resulting in no crosstalk.
Also, in the above-mentioned invention, a photodevice arranged on a path where the optical signal is not entered into the movable mirrors except the second movable mirror may be further provided, and the controller may control, during the path switchover, so as to prevent the optical signal from being entered into the movable mirrors except the second movable mirror based on the optical signal received by the photodevice
Thus, the controller can control the angle of the first movable mirror more accurately.
Also, the present invention comprises: a plurality of input ports, a plurality of input ports, input movable mirrors respectively receiving optical signals from the input ports to establish paths between the input ports and the output ports, and output movable mirrors transmitting the optical signals through the input movable mirrors to the output ports, wavelengths of the optical signals entered into the input movable mirrors may be different from wavelengths of the optical signals entered into the input movable mirrors adjoined.
As described referring to FIGS. 19A-19C, the crosstalk from the input fiber adjoined becomes the largest. Especially, when the wavelengths are the same and the phases coincide with each other, the crosstalk becomes a coherent crosstalk to have a significant influence on the optical signal. According to the present invention, the coherent crosstalk can be avoided.
Also, in the above-mentioned present invention, the wavelengths of the optical signals entered into the input movable mirrors may be different from the wavelengths of the optical signals entered into the input movable mirrors which adjoin the input movable mirrors adjoined.
Thus, the coherent crosstalks from the adjoined path and the path which further adjoins the path adjoined can be avoided.
The present invention further comprises: a plurality of input ports, a plurality of output ports, input movable mirrors receiving optical signals from the input ports to establish paths between the input ports and the output ports, and output movable mirrors respectively outputting the optical signals from the input movable mirrors to the output ports, the output movable mirrors may be separated into a plurality of areas to be arranged, and further may be arranged in each area so that a path switchover between two output movable mirrors is performed without the optical signal crossing other output movable mirrors.
Namely, the output movable mirrors respectively output the optical signals from the input ports to the output ports. The output movable mirrors are separated into a plurality of areas to be arranged, and arranged so that a path switchover between arbitrary two output movable mirrors which belong to each area is performed without the optical signal crossing other output movable mirrors.
Thus, when the path switchover of changing the output movable mirror to the output movable mirror within the same area is performed, the path switchover can be performed without the optical signal crossing the other output movable mirrors, so that no crosstalk occurs.
Also, in the above-mentioned invention, output movable mirrors not corresponding to the output ports may be further included.
When existing output movable mirrors arranged in Nxc3x97N are used for example, there is a case where the signal light between arbitrary two output movable mirrors belonging to each area can not be moved without crossing the other output movable mirrors.
Therefore, when the signal light between the arbitrary two output movable mirrors can be moved by crossing a certain output movable mirror, the output movable mirror is not used without being made correspond to the output ports. Thus, the existing output movable mirrors arranged in Nxc3x97N can be used.